The ridge waveguide and the selectively-buried ridge (SBR) waveguide are the dominant structures used for making index-guided (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P visible semiconductor lasers. The ridge-etching is a critical fabrication step for these devices, as the etch should terminate within a few hundred nanometers of the active region of the laser. To simplify this critical procedure, an etch stop layer, constructed of a material which etches very slowly compared to the surrounding AlGaInP cladding layer material, is often incorporated into the layer structure. A further advantage of an etch stop layer is that the etch depth uniformity is determined by the very low nonuniformity associated with epitaxial growth, rather than the greater nonuniformity associated with the etching process.
Etch stop layers have already been reported and used to make index-guided AlGaInP lasers. See for example:
T. Tanaka, S. Minagawa and T. Kajimura, Transverse-mode-stabilized ridge stripe AlGaInP semiconductor lasers incorporating a thin GaAs etch-stop layer, Appl. Phys. Letts. 54, 1391 (1989), PA1 K. Kobayashi, Y. Ueno, H. Hotta, A. Gomyo, K. Tada, K. Hara and T. Yuasa, 632.7 nm CW operation (20.degree. C. ) of ALGaInP visible laser diodes fabricated on (001) 6.degree. off toward [110] GaAs substrate, Jpn. Jour. Appl. Phys. 29, L1669 (1990), and Y. Ueno, H. Fujii, H. Sawano and K. Endo, Stable 30 Mw operation at 50.degree. C. for strained MQW AlGaInP visible laser diodes, Electronics Letts. 28, 860 (1992).
Demonstrated etch-stop (ES) layer materials include AlGaAs, GaAs, and (AlGa).sub.0.5 In.sub.0.5 P, all of which are lattice matched to the GaAs substrate. Of primary concern are the composition selectivity of the etch, along with optical transparency, so that optical absorption loss is not introduced into the laser stripe. Thus, although GaAs is essentially unetched by the usual etchants for AlGaInP, a GaAs ES layer must be made very thin (on the order of 1 nm) in order to quantum-shift its absorption edge to a wavelength around 650 nm or shorter. Likewise, a lattice-matched Ga.sub.0.5 In.sub.0.5 P ES layer must also be made very thin, for optical transparency. This compromises its effectiveness as an ES layer, however, since its etch selectivity is not so great. Although higher-bandgap AlGaAs and (AlGa).sub.0.5 In.sub.0.5 P can also be used as an ES layer, it is difficult to find etchants with the requisite high selectivity, and the aluminum-containing ES layer can make epitaxial regrowth difficult. Furthermore, in case of (Al,Ga)As, one prefers to avoid arsenide-phosphide interfaces in growth.